I am not sure I understand your question. I think you give way to much weight to the dispatcher's comment. Whether it was an instruction, recommendation, order or suggesstion does not matter. Zimmerman was not breaking a law and to the best of our knowledge, had lost sight of Martin at the point where the dispatcher told him that "we don't need you to follow him", at least he indicates as much in his call.
Next, you are incorrectly assuming that Martin ran away. I could just as logically conclude from the evidence presented that Martin was hiding with the intent to assault Zimmerman. Martin had plenty of time to run to his house. It could reasonably be argued that this would be what most reasonable people would do in a similar cir stance. If you cannot now prove that Martin did not assault Zimmerman, then I don't see have you have met your burden of proof beyond a reasonable doubt that it was not self defense.
One more point from your comment, you note that Zimmerman did not witness Martin commit a criminal act or perform criminal activity. If he had, would he have been justified in then following Martin? The reason I bring this up is because you are falling for exactly what the prosecution is trying to sell: that hindsight should be sufficient to convict Zimmerman. The prosecution has to prove two things which they have woefully failed to do and that is show the elements of 2nd degree murder (and now manslaughter) and that Zimmerman had no cause for self-defense as defined by Florida law. Because they cannot prove the elements, they try to confuse the jury with witness statements that testified without any certainty as to what they saw and irrelevant information such as the purchase of ice tea and skittles.
Lets presume for a second that Zimmerman racially profiled Martin, went after him with the intent to kill him, had his gun drawn and everything, Martin then jumps out of nowhere knocking Zimmerman down and the proceeds to beat the out of him to the point that a reasonable person would conclude (subjective conclusion) that their life is in danger or they could suffer serious bodily injury. Would his claim of self-defense then be justified? My point is that what really matters in this case, and I am focusing on Self-Defense not the second degree charge, is to forget all the irrelvant facts of how good or bad Zimmerman and Martin are. Can you reasonably believe that Zimmerman felt his life was ever in danager or he was about to suffer serious bodliy harm. If you cannot prove beyond a reasonable doubt that Zimmerman did not feel either of these two things during their confrontation, self-defense has been met.
Finally, I disagree with your facts. You say that after the dispatcher told him that "we don't need you to follow him" that Zimmerman continues to do it anyway. I hear Zimmerman say "okay" ant then proceed to tell the dispatcher that Martin got away and where to have the cops meet him. I take it as Zimmerman had stopped following Martin and was returning to his car. Its not the defense's burden to prove it, can the prosecution prove that Zimmerman continue the pursuit?